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The intestinal immune system is tailored to fight pathogens effectively while tolerating the indigenous microbiota. Impairments of this homeostatic interaction may contribute to the etiology of various diseases including inflammatory bowel diseases. However, the molecular architecture underlying this complex regulatory interaction is not well understood. Here, we show that the fruit fly Drosophila melanogaster has a multilayered intestinal immune system that ensures strictly localized antimicrobial responses. Enterocytes, a major cell population of the intestine, produced antimicrobial peptides (AMPs) in a FoxO- but not NF-κB-dependent manner. Consequently, animals impaired in FoxO-mediated signaling had a significantly lowered resistance to intestinal infections; they were unable to increase the expression of AMP genes and males showed an increased bacterial load in response to an infection. Conventional innate immune signaling converging onto NF-κB activation was operative in only a few regions of the intestine, comprising the proventriculus, copper cells, and intestinal stem cells. Taken together, our results imply that danger-mediated as well as conventional innate immune signaling constitute modules that contribute to the fruit fly’s intestinal immune system. We propose that this special architecture ensures localized and efficient antimicrobial responses against invasive pathogens while preserving the microbiota.

GFP has established itself as a highly useful tool throughout many areas of modern biology. Recently, the novel fluorescent protein drFP583, also termed DsRed or RFP, was cloned from a coral of the Discosomagenus. The protein is only weakly homologous to GFP and has a red emission spectrum, which makes drFP583 an attractive candidate for in vivo double labeling together with GFP variants. However, wildtype drFP583 has several drawbacks, including inefficient folding of the protein, extremely slow maturation of the chromophore, and tetramerization even in dilute solutions. Here we report on important improvements to this reporter that lead to higher levels of fluorescent drFP583 species in the cell. We further characterized our best mutant for applications in yeast and mammalian cell biology.

The endoplasmic reticulum (ER) of the yeast Saccharomyces cerevisiae contains of proteolytic system able to selectively degrade misfolded lumenal secretory proteins. For examination of the components involved in this degradation process, mutants were isolated. They could be divided into four complementation groups. The mutations led to stabilization of two different substrates for this process. The mutant classes were called ‘der’ for ‘degradation in the ER’. DER1 was cloned by complementation of the der1–2 mutation. The DER1 gene codes for a novel, hydrophobic protein, that is localized to the ER. Deletion of DER1 abolished degradation of the substrate proteins. The function of the Der1 protein seems to be specifically required for the degradation process associated with the ER. The depletion of Der1 from cells causes neither detectable growth phenotypes nor a general accumulation of unfolded proteins in the ER. In DER1‐deleted cells, a substrate protein for ER degradation is retained in the ER by the same mechanism which also retains lumenal ER residents. This suggests that DER1 acts in a process that directly removes protein from the folding environment of the ER.

Tub4p is a novel tubulin found in Saccharomyces cerevisiae. It most resembles gamma‐tubulin and, like it, is localized to the yeast microtubule organizing centre, the spindle pole body (SPB). In this paper we report the identification of SPC98 as a dosage‐dependent suppressor of the conditional lethal tub4–1 allele. SPC98 encodes an SPB component of 98 kDa which is identical to the previously described 90 kDa SPB protein. Strong overexpression of SPC98 is toxic, causing cells to arrest with a large bud, defective microtubule structures, undivided nucleus and replicated DNA. The toxicity of SPC98 overexpression was relieved by co‐overexpression of TUB4. Further evidence for an interaction between Tub4p and Spc98p came from the synthetic toxicity of tub4–1 and spc98–1 alleles, the dosage‐dependent suppression of spc98–4 by TUB4, the binding of Tub4p to Spc98p in the two‐hybrid system and the co‐immunoprecipitation of Tub4p and Spc98p. In addition, Spc98–1p is defective in its interaction with Tub4p in the two‐hybrid system. We suggest a model in which Tub4p and Spc98p form a complex involved in microtubule organization by the SPB.

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